Evolutionary principles for cooperation that have been developed from studies of diverse social organisms should apply to humans. The more immediate roots to human cooperation and conflict also may be seen in primates. However, there are challenges in studying humans and their close relatives. Objectivity is essential. There are many possibilities for study techniques (such as the questionnaire or survey) in humans, but these also offer many opportunities for confusion. One powerful approach to studying human cooperation is to look at what humans do and what the outcomes are, just as one might do for other social animals. This technique can be particularly informative when the human group lives in ways consistent with humans over most of their evolutionary past. The Dogon people of Mali, reported on by Beverly Strassmann in Chapter 14, are millet-and-onion-farming agriculturalists who do not use contraception, adhere largely to indigenous religions, practice polygyny, and have high mortality rates. In a 25-year-longitudinal study, Strassmann has investigated the hypothesis that the Dogon are cooperative breeders, where some individuals help rear nondescendent kin rather than their own progeny. She does not find that the data support this hypothesis. First, neither women nor men delay reproduction in order to raise siblings. Although parents force daughters to care for extra siblings, this is better viewed as parental manipulation because the presence of siblings reduces survivorship. Similarly, grandmothers do not appear to be effective alloparents. Rather than increasing survivorship, the presence of paternal grandmothers does the opposite, doubling the hazard

of death for a child. What matters most for survival is the presence of the mother, and other relatives are not adequate replacements. Task cooperation occurs within the groups that work and eat together, but conflict is always present in ways that Strassmann carefully explains.

In an overview of vertebrate interactions, Dorothy Cheney demonstrates in Chapter 15 that animals ranging from chickadees to chimpanzees are aware of their own status, and that of their companions, and behave accordingly. Eavesdropping on how individuals interact with others can change behaviors. Relatednesses are often known and impact interactions. In vervet monkeys, for example, an individual who has been attacked may turn and subsequently attack a relative of her opponent. Dominance hierarchies also impact such interactions. But some animal interactions are more subtle. Ravens are more likely to cache food in hidden sites when competitors are present, for example. However, the calculations of gain, cost, and punishment necessary for reciprocal altruism (here called contingent altruism) seem largely lacking outside of humans. Instead, there is a great deal of tolerance in interactions and a lack of direct payback among close relatives and long-time partners. Yet it is in these relationships where cooperation overwhelmingly occurs. A common feature of cooperative acts is that they are not necessarily transitive. Some individuals consistently take on the risky jobs, be it male chimps patrolling their territorial edges or female lions leading the hunt. This is also true in organisms (such as wasps) with much simpler brains, where cooperation flows from workers to the queen.

Observations of humans and primates in natural situations can teach us much about behavior, but environmental complexity can make causation difficult to discern. An alternative is to examine choices made under highly regulated circumstances. To address social acts such as generosity, trust, fairness, and punishment, many purportedly relevant games have been applied to humans, one simple example being the Dictator Game that allows a subject to decide whether to share a quantifiable resource with an unseen other. [This game typically yields donations of 20–30% of the resource.] Although such games have weaknesses, they seem to indicate that humans are willing to donate but only at levels indicating they consistently value themselves most highly. These and other experiments further indicate that humans favor relatives, long-term partners, and group members over outsiders, and they will suffer costs to punish cheaters. As described by Joan Silk and Bailey House in Chapter 16, versions of social games involving food or tools that likewise have been used with primates produce complex results. Cooperation clearly occurs and tracks levels of sociality in the groups, but some results are controversial and remain open to alternative interpretations.

In the modern world, most of a person’s material possessions are items that no individual could possibly make by herself. Instead they were produced with the learned and specialized expertise of others. In Chapter 17, Robert Boyd and colleagues argue that learning from others (and not intelligence alone) is the key to human success, the characteristic that has made us so adaptable. Initially in human history, most adaptations involved direct climatic protection, food acquisition, and food storage. Thus, the sharing and acquiring of information from others is a particular kind of intelligence. Boyd and his coauthors argue that cultural learners have an advantage because they can grasp the best from the past even if they innovate personally only occasionally. Tools and customs certainly make life for humans easier or possible.

The study of cooperation and conflict has come a very long way from the time, almost 50 years ago, when Hamilton (1964a,b) first pondered how to explain the evolution of worker behavior in social insects with a strange genetic system. Such analyses have spread out taxonomically, extending even to microbes. They have deepened mechanistically as we probe the molecular and genetic basis of cooperative phenomena. The findings are also beginning to show practical applications, as in medicine, and they have proven essential for understanding the structure of life, from cells to multicellular organisms to societies. Not least, study of the complex mix of cooperation and conflict helps us to understand what makes the human animal both ordinary and remarkable.

Biodiversity--the genetic variety of life--is an exuberant product of the evolutionary past, a vast human-supportive resource (aesthetic, intellectual, and material) of the present, and a rich legacy to cherish and preserve for the future. Two urgent challenges, and opportunities, for 21st-century science are to gain deeper insights into the evolutionary processes that foster biotic diversity, and to translate that understanding into workable solutions for the regional and global crises that biodiversity currently faces. A grasp of evolutionary principles and processes is important in other societal arenas as well, such as education, medicine, sociology, and other applied fields including agriculture, pharmacology, and biotechnology. The ramifications of evolutionary thought also extend into learned realms traditionally reserved for philosophy and religion.

The central goal of the In the Light of Evolution (ILE) series is to promote the evolutionary sciences through state-of-the-art colloquia--in the series of Arthur M. Sackler colloquia sponsored by the National Academy of Sciences--and their published proceedings. Each installment explores evolutionary perspectives on a particular biological topic that is scientifically intriguing but also has special relevance to contemporary societal issues or challenges. This book is the outgrowth of the Arthur M. Sackler Colloquium "Cooperation and Conflict," which was sponsored by the National Academy of Sciences on January 7-8, 2011, at the Academy's Arnold and Mabel Beckman Center in Irvine, California. It is the fifth in a series of colloquia under the general title "In the Light of Evolution." The current volume explores recent developments in the study of cooperation and conflict, ranging from the level of the gene to societies and symbioses.

Humans can be vicious, but paradoxically we are also among nature's great cooperators. Even our great conflicts-wars-are extremely cooperative endeavors on each side. Some of this cooperation is best understood culturally, but we are also products of evolution, with bodies, brains, and behaviors molded by natural selection. How cooperation evolves has been one of the big questions in evolutionary biology, and how it pays or does not pay is a great intellectual puzzle. The puzzle of cooperation was the dominant theme of research in the early years of Darwin's research, whereas recent work has emphasized its importance and ubiquity. Far from being a rare trait shown by social insects and a few others, cooperation is both widespread taxonomically and essential to life. The depth of research on cooperation and conflict has increased greatly, most notably in the direction of small organisms.

Although most of In the Light of Evolution V: Cooperation and Conflict is about the new topics that are being treated as part of social evolution, such as genes, microbes, and medicine, the old fundamental subjects still matter and remain the object of vigorous research. The first four chapters revisit some of these standard arenas, including social insects, cooperatively breeding birds, mutualisms, and how to model social evolution.

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